Design View?

When I was looking at MVVM toolkits I found this toolkit and Light MVVM toolkit. One feature I liked about Light was the feature of "blendablility" where you can have a design mode and a live mode.

This allows you to load up blend and style up your data using this design mode data. I see that MVVM uses mocks what I thought would be the same thing but after watching the video I did not see any indication that the mocked data would be displayed in design
view.

<div><font face="Verdana"><span style="font-size:15px">As far as I can tell, Simple MVVM Toolkit supports blendability in much the same way as MVVM Light, which is via a IsInDesignMode property on the base ViewModel
class. To show design-time data, just check the property and return mock data if it is true.</span></font></div> <blockquote type="cite"> <div></div> </blockquote>

<div><font face="Verdana"><span style="font-size:15px">As far as I can tell, Simple MVVM Toolkit supports blendability in much the same way as MVVM Light, which is via a IsInDesignMode property on the base ViewModel
class. To show design-time data, just check the property and return mock data if it is true.</span></font></div> <blockquote type="cite"> <div></div> </blockquote>

Where would I do this check. From the video I seen you only hook up a mock object but now switching between mock and real.